Chapter 6 - Lipids, Membranes, and the First Cells

plasma membrane
- serves as a selective barrier
- contains the reactants needed for processes, causing them to collide more frequently, allowing chemical reactions to more effective

insoluble in water due to their high proportion of nonpolar bonds (a lot of C-H bonds)
fatty acid — simple lipid, hydrocarbon chain bonded with a polar carboxyl functional group (O=C-OH or COOH)
double bonds between carbons in a hydrocarbon chain cause “kinks”
- saturated — only single bonds between carbons (saturated with hydrogen aka max number of hydrogens)
- unsaturated — one or more double bonds exist

family of lipids with a four-ring structure with an isoprenoid tail attached
- has a tiny polar top attached to the rings
- ex: cholesterol, cortisol
variation is caused by the side groups in and attached to the rings

three fatty acids linked to glycerol (3-carbon molecule)
primary role is energy storage due to the higher ratio of C-C and C-H bonds (high potential energy) than in carbohydrates
fats form when a dehydration reaction occurs between a glycerol hydroxyl group and the carboxyl group of a free fatty acid
- forms an ester linkage

consist of a glycerol linked to a phosphate group and two hydrocarbon chains of either isoprenoids or fatty acids; the phosphate is also bonded to a small polar organic molecule
- bacteria/eukarya: fatty acid tails, archae: isoprenoid tails
amphipathic — contain both hydrophillic and hydrophobic regions

amphipathic lipids form 2 types of structures spontaneously
- micelles — tiny spherical aggregates of free fatty acids
- lipid bilayer — lipid molecules align in paired sheets
liposomes — artificially generated membrane-bound vesicles

selective permeability — some substances cross a membrane more easily than other substances
- small nonpolar molecules (O2, CO2) move across quickly
- then small, uncharged, polar molecules
- then large, uncharged polar molecules
- charged ions need proteins to transport them across

when unsaturated hydrocarbon tails are packed into a lipid bilayer, kinks from double bonds produce spaces among the tails, reducing van der Waals forces, weakening the barrier
packed saturated hydrocarbon tails have fewer spaces, more van der Waals forces
as length of the saturated hydrocarbons increase, forces holding them together also increase, making it even desner
at lower temperatures, membranes become even less permeable

diffusion — spontaneous movement of molecules and ions
concentration gradients causes net movement of a solute to move away from regions of high concentration
- diffusion down a concentration gradient
- results in an increase in entropy
passive transport — when substances diffuse across a membrane without an outside energy source

osmosis — diffusion of water
only occurs when solutions of different solute concentrations are separated by a membrane that permits water to cross, dilutes the higher concentration of solute
- solutions on both sides of the membrane experience a change in volume as well as a change in solute concentrations
- opposing forces, such as pressure from gravity, exert resistance to the net movement of water
effects of osmosis
- if solution outside of cell is hypertonic relative to inside of cell, water flows out of the cell and it shrinks
- if solution outside is hypotteronic relative to the inside of cell, water flows into the cell and it swells or bursts
- is the solution is isotonic, there is no net movement of wa

fluid (mosaic model) — some proteins span the membrane
integral membrane/transmembrane proteins — have segments facing both the interior and exterior of cell
peripheral membrane proteins — bind to membrane lipids or integral membrane proteins without passing through
ion channels — specialized transmembrane proteins